Surgical navigation systems are increasingly used as aids in surgical procedures. Generally, a surgical navigation includes a tracker, a localizer and a processor. The tracker is attached to an instrument or section of tissue the position of which is to be tracked. The localizer, relative to the tracker, is static. One or more transmitters are contained in either the tracker or localizer. The other of the localizer or the tracker contains one or more complementary receivers able to detect the energy emitted by the transmitters. It is known to construct surgical navigation systems out of transmitter receiver pairs wherein the transmitted energy is photonic energy, (visible light, UV and/or IR), sonic energy, electromagnetic energy or RF energy. The processor receives signals from the receiver(s) indicating the strength of the energy emitted from the transmitter(s) or other position/orientation dependent characteristic. Based on these signals, the processor generates data representative of the position and orientation of the tracker relative to the localizer. By inference, this leads to the position and orientation of the body tissue or instrument to which the localizer is attached. Often this information is presented on a display connected to the processor.
There are a number of reasons why, in a surgical procedure, it is desirable to track the position of body tissue. In an orthopedic surgical procedure, for example, it is desirable to track the position of hard tissue, bone. This tracking is often performed as part of a procedure to replace a joint such as knee, hip or shoulder. Prior to the replacement of the original joint, it is desirable to track the motion of the bones connected by the joint. For example, in a knee replacement procedure, the surgeon will want to know the relative position and range of motion of the below the knee tibia to the above the knee femur. During the actual joint replacement process, this information helps the surgeon fit the replacement joint to the bone so that, post procedure the patient's bones are properly aligned relative to each other and the bones have the appropriate range of motion.
In other surgical procedures, it is useful to know the position of the patient's tissue in order to assist in the placement and/or control of a surgical instrument at or near the surgical site. In this type of procedure, the system tracks the location of the patient's tissue and the surgical instrument. Based on these data, the processor generates a map that indicates the position of the surgical instrument relative to the tissue or an adjacent surgical site. The surgeon, by reference to this map, properly positions the instrument to accomplish the desired surgical task. Some surgical navigation systems are integrated with the units that regulate the actuation of the surgical instrument. Some versions of these integrated systems are constructed so that, based on the map data indicating the position of the surgical instrument relative to the tissue, actuation of the instrument is regulated.
As mentioned above, the tracker-localizer pair of a surgical navigation system exchanges one of a specific form of energy. Many currently available surgical navigation systems are designed so that their trackers emit and localizers receive photonic energy such as infra red light. These systems often typically require trackers that are relatively large in size, surface areas of 4 cm2 or more.
When a tracker is attached to tissue, it must firmly be attached to tissue site it is intended to track. This is because, if the tracker moves relative to the tissue, the system may not generate signals that accurately represent tissue position. Currently, in order to track the position of bone with an IR tracker, the following protocol is employed. A hole is drilled in the bone. A post is fitted into the hole so it is firmly attached to the bone. Often, to accomplish this latter intermediate goal, it is necessary to secure the post to the bone so it extends through the opposed sides of the bone. Once the post is firmly secured in place, the tracker is mounted to an exposed end of the post. Having to so mount the tracker to the bone appreciably adds to the trauma to which the patient is exposed when required to undergo a surgical procedure. This is especially true when, in order to prevent the post from moving, it is necessary to extend the post through the bone.
Moreover, in this type of arrangement, the post and tracker sub-assembly typically extend 10 cm or more above the patient. Given the rather large size of the tracker, this sub assembly, while serving as an important aid in surgery, also functions as an obstruction the surgical personnel have to take care to avoid.
To avoid the above discussed disadvantages of conventional IR surgical navigation systems, there has recently been proposed a system that relies on electromagnetic navigation. This system relies on relatively small fiducial markers designed to be implanted in the bone subcutaneously. Given the relatively small size of these markers, when fitted to the bone, there is no need fit them through the bone. Thus, use of these markers is expected to result in less trauma to the patient and reduced clutter adjacent the surgical site. These markers are intended to exchange EM signals with complementary localizers located adjacent the patient.
While the above proposed system offers some benefits, there are some limitations associated with its use. Specifically, the system introduces into the operating room localizers with relatively large antennas, coils. These structural members are used to both inductively transfer energy to and exchange signals with the components internal to the fiducial markers.
Moreover, the signals exchanged between the fiducial markers and the complementary coils are electromagnetic signals. Thus, the strength and direction of the signals are affected by the presence of ferromagnetic materials in the path between the coils and markers. To ensure a surgical navigation system of this variety generates data that accurately indicates the positions of the fiducial markers, and the bones to which they are attached, it is necessary to ensure that space between the coils and markers are free of ferromagnetic materials or other objects that can distort the transmission of the EM energy. This may mean, for example, that instruments formed with ferromagnetic materials should not be introduced into the space during the tracking process. Such instruments include, but are not limited to, powered surgical tools with energized stators. This requirement limits the utility of this system.